1. Field of the Invention
The present invention relates to a process suitable for formation of a functional deposited film containing silicon, especially a polycrystalline silicon-containing deposited film usable suitable for semiconductor devices, photosensitive devices for electrophotography, line sensors for inputting of images, image pick-up devices, etc.
2. Related Background Art
For example, for formation of amorphous silicon films, there have been tried various processes such as vacuum evaporation process, plasma CVD process, CVD process, reactive sputtering process, ion plating process, photo CVD process, etc. and in general the plasma CVD process has been widely employed and commerciallized.
However, deposited films comprising amorphous silicon must be further improved in overall characteristics on electrical and optical characteristics, fatigue characteristics, upon repeated uses, environmental characteristics and productivity and mass-productivity including uniformity and reproducibility.
Especially, for some uses, electrical characteristics, enlargement of area, uniformization of film thickness and uniformity of film quality must be fully satisfied and besides mass-production with high reproducibility must also be attained by high-speed film formation. For this purpose, a heavy equipment investment is required for apparatuses for mass-production in formation of amorphous silicon deposited films by the plasma CVD process. Moreover, control items for the mass-production become complex resulting in less control and further, fine adjustment of apparatuses is necessary. Therefore, these points are further problems to be improved.
On the other hand, normal pressure CVD or LPCVD process have been used for formation of polycrystalline deposited films of silicon, but these processes require high temperature and so substrate materials are limited. Besides, there are difficulties in control of crystal grain size, and of grain size distribution, compensation of defects present in grain boundaries or arrangement of crystal faces. For these reasons, there are also the problems that satisfactory film characteristics cannot be obtained, enlargement of area is difficult, uniform film thickness and quality cannot be obtained, and reproducibility is inferior.
In the plasma CVD process, temperature of substrate can be lowered as compared with the normal pressure CVD process or LPCVD process, but structure of reaction apparatuses is limited to form a stable plasma and further, there are many other parameters (kind of introduction gases, flow rate of gas, pressure, radio frequency power, exhaust velocity, etc), which have great effects on plasma. Therefore, plasma subtly varies due to a slight variation of these parameters or a combination of them, often resulting in adverse effects on uniformity, and electrical and optical characteristics. Furthermore, bombardment of electrons or ions present in plasma against deposited films causes much damage, which is one of factors of deterioration of the film characteristics.
As explained above, at present, both the amorphous silicon film and polycrystalline silicon film have problems which are due to their process of formation. Especially, from the points of improvement and stabilization of electrical characteristics of deposited films, in formation of polycrystalline silicon films, development of a process for forming a polycrystalline silicon film of a large area at low cost with keeping practicably sufficient characteristics, uniformity, and stability has been demanded.
The photo-CVD process has been proposed for solving the above problems, but according to the conventional photo-CVD process, it is difficult to produce a polycrystal deposited film while controlling crystal grain side and grain size distribution, electrical characteristics of grain boundaries are insufficient, and besides control of face direction is also difficult. Further, the photo-CVD process has limitation in usable raw material gases, and this process is not profitable in productivity and cost.